A mating problem for Nassau groupers

Brice Semmens, Reef Environmental Education Foundation (REEF)

Brice – The Nassau grouper is one of the larger Caribbean reef fish. It’s a tiger-striped fish and it’s really a Caribbean icon. It’s one of those fish that you often seen framed in the wish-you-were here postcards that you may get from your friends in the Caribbean.

It’s something that people really enjoy seeing on the reefs both because of its beauty and also because when you find a large fish like that on a reef it makes that reef feel a little bit more wild.

So typically Nassau grouper are solitary and territorial. So they defend areas of reefs. Once or twice during the year, shortly after the winter full moons, they get randy and they go about looking for mates.

The way that Nassau grouper do is, they will often swim over very large areas in order to get to one specific place on a reef. And many, many individuals will do that swim and gather together in these large aggregations or groups.

And during that winter full moon they will all spawn together. And when all’s said and done they’ll head back to their home reefs, back to their territories and go about their business.

Helen – Being so predictable in time and space, these spawning sites have historically been a major target for fisheries across the Caribbean. And that’s causing serious problems for the Nassau grouper, isn’t it?

Brice – There are lots of places now where they have been basically fished to exhaustion, they’ve been fished to the point where they’re no longer worth fishing by the local fishermen.

Basically the way that that has proceeded is through fishing down these spawning aggregations. They are literally being serially depleted. And you can imagine when you get very valuable large fish and you get a whole bunch of them all in one location that’s really an irresistible draw to fishermen. And so, when these aggregations are found, they’re hit very hard until all of the fish that are there are virtually gone.

And what’s really kind of scary about it is that in most of these places where this really heavy depletion or overfishing on spawning aggregations has happened there’s very little evidence that recovery happens after the fact. So once you’ve fished out those spawning aggregations there’s virtually no evidence that they recover.

So part of the reason that we’re doing research down in the Cayman Islands is that, well, first of all it’s one of the last places in the Caribbean that has a large healthy aggregation of Nassau groupers. But also it represents a great opportunity to start to investigate how aggregations happen. So getting some basic information on the natural history of Nassau grouper spawning aggregations. And also why it is that when these aggregations are fished to the pint of near exhaustion they don’t recover.

Helen – And you’ve got this wonderful site, by the sounds of it, in the Cayman Islands where you do still have a large aggregation that’s forming. How big is that aggregation? How many fish are we seeing there?

Brice – Well, as you can imagine, when you’re trying to count fish on a very deep site it can be a challenge. So we’re using a series of different techniques to try and get the answer to that question.

We’re estimating somewhere in the ballpark of 3000 individuals. And to put that into perspective, when that aggregation was first found on the west end of Little Cayman, there was something like 7000 individuals.

And at the time that it was discovered by local fishermen it wasn’t protected. And in the first two years, nearly half of the fish on that aggregation were fished out.

But the Cayman Islands was able to make a very, very rapid decision to conserve not just that spawning site, but in fact all of the spawning sites around the Cayman Islands - just happens that most of them had already been fished out so they’re protecting these legacy sites. And so that’s where the Reef Environmental Education Foundation came in, the REEF programme, and we started working with the Cayman Islands government, to, after the protections went into place, try to determine whether or not those protections were a) effective, and b) something that should be kept into the future, moving forward as a management conservation programme.

One of the basic research questions that we had is are these local fish? Where are these fish coming from? Where are they going to afterwards?

And it turns out that, based on the tagging work that we did, not only are all of the fish on the west end of Little Cayman that are showing up to spawn Caymanian fish, but they are in fact all only from the island of Little Cayman. And so we found that despite tagging in all three of the Cayman Islands, Grand Cayman, Cayman Brac, and Little Cayman, we found no evidence of any fish moving between islands over deep water.

What’s also really important is that it turns out that basically every single reproductive-aged fish on the island of Little Cayman goes to the west end spawning site during the spawning season.

And so when you are fishing that spawning site you are basically exposing your entire sum reproductive potential for your spawning stock of fish, your whole population of fish, to harvest pressure.

Helen – It seems to make nothing but common sense that spawning aggregations should be protected. Is that something that we’re seeing outside the Caymans? Are other spawning sites being protected from fishing pressure?

Brice – So one of the nice things that’s happening throughout the Caribbean is that slowly but surely regional governments are sort of understanding that these spawning sties are important. Not just for Nassau grouper, but it turns out that all kinds of different species use the exact same location on the reef. For whatever reason, there’s an importance of place where these spawning aggregations actually take place.

And so for the Caymans, for instance, so far we’ve documented something like 25 different reef fish species that are using the spawning site to reproduce. And so there is a movement to conserve these locations spatially to establish Marine Protected Areas.

In addition, for grouper species in particular there are efforts - for instance in the Bahamas and Bermuda - to disallow selling of grouper in restaurants and grocery stores during the time of year when they’re reproducing. And it’s a way of controlling the fishery during this important time.

Now it’s important to know that no Nassau grouper has ever been seen to reproduce when they aren’t at the spawning site. And what that means is that when they’re spawning at the spawning site that is the only time during the year that they’re ever reproducing.

So if you mess with that event you are cutting off, effectively, any reproduction in the population, which is a very dangerous thing to do.

Helen – Is this also a global problem that we’re seeing in terms of the depletion and disappearance of spawning aggregations?

Brice – Spawning aggregations have, you know, it’s something that’s been going on for years, fishing spawning aggregations and fishing locations where fish get together in order to reproduce. There’s big examples that people maybe don’t really realise but cod is a great example in the Atlantic. Bluefin tuna in the Gulf of Mexico, that’s a spawning aggregation. Fish from multiple stocks of bluefin tuna from as far away as the Mediterranean will come to the Gulf of Mexico in order to reproduce and there they are targeted for harvest.

So it’s a common problem in fisheries in a global context and it represents both a management challenge and an opportunity once we understand reproductive behaviour of these species to conserve that reproductive potential of the population during very vital times, during their natural history.

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